SB 



U. S. DEPARTMENT OF AGRICULTURE. 

BUREAU OF PLANT- INDUSTRY— BULLETIN NO. 146. 



B. T. Galloway, Chief of Bureau. 



THE SUPERIORITY OF LINE BREEDING 
OVER NARROW BREEDING. 



O. F. COOK, 
BioNOMisT, Bureau of Plant Industry. 



Issued April 29, 1909. 




WASHINGTON: 

GOVERNMENT PRINTING OFFICE, 
1909. 




Pass 8 M I 2 ?j 
Book 'C 7 



Digitized by tine Internet Archive 
in 2010 with funding from 
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U. S. DEPARTMENT OF AGRICULTURE. 

BUREAU OF PLANT INDUSTRY— BULLETIN NO. 146. 

B. T. (tALLOWAy, Chief of Bureau. 



f f 



THE SUPERIORITY OF LINE BREEDING 
OVER NARROW BREEDING. 



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or F/ COOK, 

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BiONOMisT. Bureau of Plant Industry. 



Issued April 29, 1909. 








WASHINGTON: 

GOVERNMENT PRINTING OFFICE, 

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BUREAU OF PLANT INDUSTRY. 



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PhysiologiKt and PaihoJotjist, and Chief of Bureau, Beverly T. Galloway. 

Physiologist and Pathologist, and Assistant Chief of Bureau, Albert F. Woods. 

Laboratory of Plant Pathology, Erwin F. Smith, rathologist in Charge. 

Fruit Disease Investigations, Merton B. Waite, Pathologist in Charge. 

Investigations in Forest Pathology, Haven Metcalf, Pathologist in Charge. 

Cotton and Truck Diseases and Plant Disease iSurrey, William A. Orton, Pathologist in 
Charge. 

Pathological Collections and Inspection ^Vork, Flora W. Patterson, Mycologist in Charge. 

Plant Life History Investigations, Walter T. Swingle, Physiologist in Charge. 

Cotton Breeding Investigations, Archibald D. Shamel and Daniel N. Shoemaker, Physi- 
ologists in Charge. 

Tobacco Investigations, Archibald D. Shamel, Wightman W. (jlarner, and Ernest H. 
Mathewson, in Charge. 

Corn Investigations, Charles P. Hartley, Physiologist in Charge. 

Alkali and Drought Resistant Plant Breeding Investigations, Thomas II. Kearney. Physi- 
ologist in Charge. 

Soil Bacteriology and Water Purification Investigations, Karl F. Kellerman, Physiolo- 
gist in Charge. 

Bionomic Investigations of Tropical and Subtropical Phints, Orator F. Cook. Bionomist 
in Charge. 

Drug and Poisonous Plant and Tea Culture Investigaliuns, Rodney II. True. Physiologist 
in Charge. 

Physical Laboratory, Lyman J. Briggs, Physicist in Charge. 

Agricultural Technology, Nathan A. Cobb, Crop Technologist in Charge. 

Taxonomic and Range Investigations, Frederick V. Coville, Botanist in Charge. 

Farm Management, William .T. Spillman, Agriculturist in Cliargc 

Cfrain Investigations, Mark Alfred Carleton, Cerealist in Charge. 

Arlington Experimental Farm and Horticultural Investigations, I.oo C. Corliett, Horti- 
culturist in Charge. 

Vegetable Testing Gardens, William W. Tracy, sr.. Superintendent. 

Sugar-Beet Investigations, Charles O. Townsend, Pathologist in Charge. 

Western Agricultural Extension, Carl S. Scofield, Agriculturist in Charge. 

Dry-Land Agriculture Investigations, E. Channing Chilcott, Agriculturist in Charge. 

Po)nological Colleciiuns, Gustavus B. Brackett, Pomologist in Charge. 

Field Investigations in Pomology, William .\. Taylor and G. Harold P(>\v<ll, i'oniologists 
in Charge. 

Experimental Gardens and Grounds, Edward M. Byrnes, Superintendent. 

Foreign Seed and Plant Introduction, David Fairchild, Agricultural Explorer in Charge. 

Forage Crop Investigations, Charles V. Piper, Agrostologist in Charge. 

Seed Laboratory, Edgar Brown, Botanist in Charge. 

Grain Standardization, John D. Shanahan, Crop Technologist in Charge. 

i^ubiropical Garden, Miami, Fla., P. .T. Wester, in Charge. 

Plant Introduction Garden, Chico, Cal., W. W. Tracy, jr.. Assistant Botanist, in Charge. 

South Texas Garden, Brownsville, Tex., Edward C. Green, Pomologist in I'harge. 

Farmers' Cooperative Demmislration Work. S<>aman .\. Knapp, Special .Vgent in Charge. 

Seed Dislrihution (Directed by Cliicf of Bureau). Lisle Morrison. .Assistant in Geneml 
Charge. 



Editor, .1. E. Rockwell. 

Chief Clerk. .Tames E. .Tones. 



]{li>.\(i.\lii' l.w i:s iicAiiovs ov T]!iii'ir.u. .\\i> Sriiriicn'HAi. Pi..\.nts. 

s('1i;nTiI'M.' s r.\ir. 

(». l'\ Cook, t.ianomist in cinniiv. 

a. N. Collins anil i'. 1.. l.i'Wlon. XssisUint liolanist.^. 

H. Pittier, .1. II. Kinslcr. ;nid .\. McLacblan. Si>c<ial .\genl.s. 

C. B. Doyb- and It. M. M.'Mdf, Srirntijir AsKi.^l„nl.y. 

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2 1 1909 "^ 

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LETTER OF TRANSMITTAL. 



U. S. Department of Agriculture, 

Bureau or Plant Industry, 

Office of Chief of Bltreau, 

Washington^ D. C.^ February 25^ 1909. 

Sir : I have the honor to transmit herewith a paper entitled " The 
Superiority of Line Breeding over Xarrow Breeding," by Mr. O. F. 
Cook, and recommend its publication as Bulletin No. 146 of the 
special series of the Bureau of Plant Industry. 
Respectfully, 

B. T. Galloway, 

Chief of Biireau. 
Hon. James Wilson, 

Secretary of Agrienlticre. 

3 
146 



CONTHNTS. 



Page. 
7 
Introduction 

Three principal types of reproduction ° 

Effects of restriction of descent - ^ 

Agricultural improvement by increase of uniformity 

Vegetative propagation the most effective method of line breeding 11 

Vegetative propagation depends on longevity 

Other methods of line breeding - - j^ 

Relation of self-fertility to vegetative propagation 15 

In-and-in breeding -_ 

Self-fertilization supplements broad breeding in natural species lo 

Broad breeding and line breeding in the same species 19 

Line breeding a system of propagation ^^ 

How selection improves line-bred varieties 

Selection a conservative process 

Uniformity not a normal condition of hereditx - 

Two forms of mass selection - - - - 

The "running out" of varieties ^ 

Rejuvenescence of varieties 

A balance between broad breeding and narrow breeding 31 

Avoidance of unnecessary uniformity 

The "fixing of characters" by line breeding ^^ 

Recurrence of diversity in crosses between line-bred groups. 36 

37 
Conclusions - 

Index - - 



146 



5 



B. r. I. — 444. 



THE SUPERIORITY OF LINE BREEDING 
OVER NARROW BREEDING. 



INTRODUCTION. 

Xone of the applications of the science of evohition to the art of 
breeding has been tlie subject of so much stuclj^, experiment, and dis- 
cussion as the relative merits of inbreeding and cross-breeding. Xev- 
ertheless, opinions remain as discordant as ever, for some experiments 
apjDear to indicate that cross-breeding is better than inbreeding, while 
other experiments seem to show quite as definitely that inbreeding 
is better than crossing. 

Exjjeriments are our means of securing answers to scientific ques- 
tions. When experiments appear to give contradictory or equivocal 
answers we know that we have not asked our questions in the right 
way. We are warned that there are more differences among our 
facts than our theories have recognized. We must find another point 
of view more favorable for the interpretation of the facts. 

Some breeders have generalized upon the results of their OAvn ex- 
periments and ignored the contrarj^ results reported elsewhere. 
Others have taken a broader view of the subject and have sought to 
compromise the issue between inbreeding and cross-breeding by as- 
suming that the different types of plants or animals are so differently 
constituted that inbreeding represents the normal method for some 
species, cross-breeding for others. The effect of this opinion has been 
merely to postpone the issue, not to determine it, for we can not avoid 
thinking of reproduction as a physiological process or fail to ask 
the question why members of one species should appear to thrive 
best by inbreeding and members of another species by crossing. 

It is the purpose of this paper to point out some of the sources of 
confusion which have interfered with definite scientific solutions of 
problems of breeding, and to indicate a point of view from which 
contradictory opinions can be reconciled. As soon as it becomes pos- 
sible to recognize physiological relations behind the apparently con- 
tradictory evidence the subject is opened to new methods of in- 
vestigation. 

7 
146 



8 LINE BREEDING AND NAREOW BREEDING. 

THREE PRINCIPAL TYPES OF REPRODUCTION. 

I. Broad breeding. 
II. Narrow breeding. 
III. Line breetliug : 

(«) In-and-in breeding. 
(1)) Self-fertilization. 

(c) Parthenogenesis. 

(d) Vegetative propagation. 

Some of the difficulties in the stuch^ of breeding have come from 
the fact that three principal types or methods of reproduction have 
been under consideration, though onl}^ two have been formally 
recognized. Many writers on breeding could be charged with the 
error of reasoning called by logicians " the undistributed middle.'" 
They have applied the same names to conditions essentially different. 

The term " cross-breeding " has been applied to the condition of 
free interbreeding among the members of large groups, such as 
species in nature, as well as to the mating of individuals representing 
small, close-bred varieties, or even to the crossing of closely related 
individuals of the same strain when such crosses are being contrasted 
with self-fertilization. 

The term " inbreeding " has also beeii used for a wide series of 
conditions. It is commonl}^ applied to the self-fertilization of plants 
by their own pollen as well as to in-and-in breeding among closely 
]"elated members of the same family group, and even to the more 
miscellaneous form of close breeding found in a flock of sheep or 
poultry when no " new blood '' is brought in. 

The meanings of the two terms remain distinct only at the extreme 
ends of the series of facts to which they are applied, and widely 
overlap in the middle. Whether a particular condition of descent 
is to be described as inbreeding or as cross-breeding is usually allowed 
to depend entirely on the particular instances in hand. The same 
case may be called " inbreeding " if the author wishes to compare it 
with another in Avhich there is wider crossing, or it may be called 
" cross-breeding *' if it is to be compared with a smaller amount of 
crossing. Thus the same word may describe different conditions, or 
different words the same condition. 

For many purjDOses of comparison the use of the words " cross- 
breeding " and " inbreeding ^ in these merely relative senses is con- 
venient and entirely pr()})or, but it leads to serious confusion when 
we attempt to distinguish the underlying factors. This confusion can 
be avoided by choosing other words to which more definite meanings 
can be attached. For the purposes of the present explanation it seems 
desirable to recognize three principal conditions or methods of repro- 
duction, which may be characterized as follows: 

140 



EESTRICTION OF DESCENT. 9 

Broad hreeding is the condition of descent found in natural species, 
which consist of millions of diverse individuals freely interbreeding 
with each other, so that the vast numbers of lines of descent of the 
species are joined into a broad network.'' 

Narrow hreeding is the condition of descent found in carefully- 
selected varieties, consisting of relativsly small numbers of closely 
similar individuals interbreeding with each other to form a narrow- 
network of descent. 

Line hreeding is the condition of descent found in strains descended 
from single individuals propagated without interbreeding with other 
lines of descent, so that no network is formed. 

The comparison of descent in normal broad-bred species to a net- 
work or fabric helps to distinguish between the two types of restricted 
descent. The usual process of selection to secure uniformity in the 
expression of characters in a sexually reproduced group is called 
" narrow breeding " because it reduces or subdivides the network of 
the species to narrow sti'ands. When descent is still further restricted 
so that lines of descent from particular individuals are kept separate 
we have the condition called " line breeding." 

Four forms of line breeding are to be considered in later sections: 
Vegetative propagation, parthenogenesis, self-fertilization, and in- 
and-in breeding. 

Broad breeding might be described as a wide and indiscriminate 
form of cross-breeding, and line breeding as the strictest form of 
inbreeding. Narrow breeding is the intermediate condition which 
many authors have called " cross-breeding " and many others " in- 
breeding," The mistake has been to suppose that this intermediate 
condition of narrow breeding is the same as one or the other of the 
extremes of the series. Narrow breeding is in reality quite as 
different from broad breeding and from line breeding as these are 
from each other, and in some very important respects more different, 
as will be shown in the later sections. 

EFFECTS OF RESTRICTION OF DESCENT. 

Broad breeding is the normal condition of reproduction in natural 
species, the condition in which normal evolution takes place. The 
lines of descent of any individual if traced backward are found to 
diverge more and more to connect with a vast number of ancestors, 
and eventually with all the members of the group, for the network 
is continuous. It is this continuous network of descent that makes 
a species a definite biological entity and not a mere collection of 

" Cook, O. F. Tlie Vital Fabric of Descent. Proceedings, Washington Acad- 
emy of Sciences, vol. 7, pp. 301-323. 1906. 
76684— Bui. 146—09 2 



10 LINE BREEDING AND N.^KOW BREEDING. 

similar individuals. The absence of any restriction of interbreeding 
among the members of a species allows the lines of descent to be 
joined into a broad network. Eestriction of descent means a limita- 
tion of the number of ancestors and of lines of descent, so that only 
a narrow network, or none at all, can be formed. 

If we recognize broad breeding as the natural condition in which 
organisms thrive and species make evolutionary progress, it is easy 
to understand that persistent narrow breeding may interfere with 
such progress or cause degeneration in characters or qualities already 
attained. Evolution, instead of being brought about by restriction 
of descent, as some have supposed, is actually hindered and finally 
undone." 

There is no evidence to show that any form of restricted descent, 
either to narrow networks or to simple individual lines, is an advan- 
tage to a group of organisms in the sense that it enables them to 
produce stronger individuals in larger numbers. The evidence all 
tends to show that restriction of descent results, sooner or later, in 
degeneration and extinction. Nevertheless, different methods of 
restricting descent may bring very different results, which the dis- 
tinction between narrow breeding and line breeding helps us to recog- 
nize. If biological phenomena were governed by rules of logic it 
could be reasoned that line breeding must be more promptly injurious 
than narrow breeding, because it is narrower than narrow breeding. 
The actual results often show the contrary. One line of descent often 
proves to be stronger than a few lines. Line breeding by self-fertili- 
zation or in-and-in breeding is often superior to more indiscriminate 
breeding among the descendants of a few ancestors. After distin- 
guishing the three general types or conditions of reproduction, it 
becomes possible to consider that line breeding is often superior to 
narrow breeding, without any need of supposing that line breeding 
is ever superior to broad breeding, in maintaining higher degrees of 
vigor and fertility. 

Vigor and fertility, though of fundamental biological and evolu- 
tionary importance, are not the only forms of superiority that the 
breeder takes into account, for many highly prized characteristics of 
domesticated varieties, such as seedlessness. are in the nature of bio- 
logical degenerations artificially preserved for the special uses of 
man. We think of even a seedless variety as fertile if it yields us 
good quantities of a useful product, though it has to be reckoned as 
completely sterile in the biological sense. But whether we wish in 

»Cook, O. F. Methods and Causes of Evolution. Bulletin 136, Bureau of 
Plant Industry, U. S. Department of A.^ricnlturo. lOOS. See also Aspects of 
Kinetic Evolution, in Proceediugs, Washington Academy of Sciences, vol. 8, 
p. 197, 1907. 
146 



VEGETATIVE PEOPAGATION. 11 

particular cases to preserve the biological superiority of our varieties 
or are willing to sacrifice vigor or fertility to special purposes of use, 
we need to recognize the underlying biological principles. We can 
then understand that different kinds of improvement are to be sought 
in different directions and by different methods. 

AGRICULTURAL IMPROVEMENT BY INCREASE OE UNIFORMITY. 

The object of restricting descent to superior individuals is to secure 
progen}^ as close as possible to the parental standard of excellence. 
This object is best attained when the progeny continue to follow each 
other in an unbroken series of individuals closely similar to the high- 
grade ancestor and closely alike among themselves. 

Increased uniformity often constitutes an agricultural improve- 
ment of a . variety quite apart from the question whether any indi- 
viduals of the variety advance beyond the standard of the carefully 
selected ancestor of the line-bred group. It has often been supposed 
that progenj^ obtained under restriction of descent are actually 
superior to any of their broad-bred ancestors, but this is now seriously 
questioned, and many recent writers have denied it altogether. The 
chief advantage gained through restriction of descent is conservative 
rather than constructive. The practical improvement of varieties by 
selection and other forms of restricted descent lies in the preservation 
of characters that already exist rather than in the attainment of new 
characters. 

VEGETATIVE PROPAGATION THE MOST EFFECTIVE METHOD OP 

LINE BREEDING. 

There can be no doubt that the most effective way of securing the 
desired uniformity and of maintaining it for long periods of time is 
by the process of vegetative propagation as applied to many culti- 
vated plants. To prove this by new experiments might require far 
longer than the lifetime of a man, but the history of agriculture 
makes such experiments unnecessary''. We know that many tropical 
root crops and other plants have been propagated from cuttings since 
very remote periods, to be counted in thousands of years. Our care- 
fully selected narrow-bred varieties of seed-propagated plants have 
been known for only a few decades at the most. The majority of them 
go out of use and disappear after only a few generations, giving place 
to other " new " varieties, better at first than the old, but giving no 
more assurance of permanent superiority. 

VEGETATIVE PROPAGATION DEPENDS ON LONGEVITY. 

To treat vegetative propagation as a form of line breeding may 
appear unwarranted in view of the current opinion that vegetative 
propagation is a purely nonsexual process, whereas self-fertilization 

146 



12 LINE BBEEDING AND N.IlEEOW BREEDING. 

and in-and-in breeding are usually viewed as forms of sexuality. It 
is true that vegetative propagation is often carried on by parts of 
plants which have no apparent connection with the organs of sexual 
reproduction, but there are other and more fundamental considera- 
tions which show that all of our higher types of plants are continu- 
ously dependent for their existence uj)on the sexual process of con- 
jugation, without regard to whether particular individuals are 
raised from seed or gi'own from cuttings.'' 

It is misleading to suppose that vegetative propagation as carried 
on by the higher plants involves a cessation of conjugation. New 
conjugations become unnecessary because the old conjugation is 
greatly prolonged. Thus vegetative propagation is not to be consid- 
ered as a substitute for conjugation, but as evidence of an ability 
of the cells of the plants to continue in a state of conjugation without 
the need of frequent renewal. This power to maintain existence by 
vegetative propagation depends upon a special propertj'' or quality 
not shown in plants whose cells are able to remain in a state of con- 
jugation only for the lifetime of a single seed-propagated individual. 
Vegetative propagation is to be viewed as a form of longevity of the 
protoplasm, enabling growth to continue without a new conjugation. 
Without this power of longevity, vegetative propagation would be as 
impossible as it would be for an annual herb to grow into a tree. 
Species and varieties differ greatly in their powers of vegetative 
propagation, just as they differ in other forms of longevity. 

Many writers have recognized that conjugation is a means of rejuve- 
nation or renewal of the energy of the protoplasm. The bodily 
activities of organisms appear greater in young plants and animals 
■and lessen with age. The renewal of the energy of the protoplasm 
by conjugation is effective in different species for very different 
periods of time. Many plants are annuals, living for only a few 
weeks or months, while others have an enormous longe\dty. The 
ages of individual trees of several different species have been reckoned 
in hundreds or thousands of years. To maintain the life of such 
organisms the process of conjugation must have a long-sustained 
ejfficiency, avoiding the need of the frequent renewal, as in vegetative 
propagation. 

The longevity of a plant capable of vegetative propagation is not 
to be measured by the life of a single individual, but by the length 
of the series of vegetative individuals that can be produced without 
renewed conjugation. Differences of longevity probably represent 

« Cook, O. F., and Swingle, W. T. Evolution of fcelluliir Structures. Bulletin 
81, Bureau of Plant Industry, U. S. Department ot Ajirieulture. 1905. 

146 



VEGETATIVE PROPAGATION. 13 

diiferences in the structure or quality of the protoplasm itself. 
Rudely, we may comj)are the organisms to clocks and say that some 
have stronger mainsprings or are more effectively wound up. In 
vegetative propagation we may say that the protoplasmic main- 
springs outlast the individual organisms and remain serviceable 
through a long series of organisms. 

Longevity is not proportional to size. The persistent vegetative 
growth of some of our herbaceous plants might enable them to excel 
the largest trees if all the tissues originating from a single seed could 
be kept together as in the case of the trees. The tree lives only as 
long as the protoplasm of the cells retains its vigor, or until it be- 
comes too large to support or nourish itself, but with a perennial 
herb the longevit}^ of the protoplasm is passed on through many 
vegetative generations. The individual plant may not remain alive 
for more than a year or two, and yet the same rootstock may creep 
along in the ground for decades or centuries. The greater longevity 
of plant organisms may depend somewhat upon the fact that they 
continue to form new tissues instead of attaining a definite maturity 
merely from the different habits of gi^owth. The weakened vitality 
of the protoplasm may be the same in the vegetative variety as in a 
tree or a long-lived animal, the difference of visible results arising 
merely from the different habits of growth. The Aveakened vitality 
of old varieties of potatoes or of sugar cane may be compared with 
the gradual weakening of aged trees or of aged men. There is a 
slackening of the organic energies which can be quickened only by 
new conjugations. 

In a species in which the individuals are short lived we think of 
a new conjugation as necessary to restore the energy of the pro- 
toplasm for the growth of each generation. With long-lived species 
or those capable of vegetative propagation it is evident that there 
is no such necessity of frequent renewal of conjugation. 

Additional insight regarding the nature of conjugation has been 
gained in recent years. Instead of conjugation being a process which 
takes place only when a new generation is to begin, it is now known 
that the cells which compose the bodies of the higher plants and 
animals all represent a state of prolonged conjugation. They are not 
like the simple cells which are formed hetween conjugations in the 
lower and more primitive groups, but are double cells like those 
which are formed in the lower groups only during conjugation. The 
complex bodies of the higher plants and animals are built up, not 
merely because conjugation takes place, but because conjugation con- 
tinues throughout the lifetime of the individual, however long this 
may be. 

146 



14 LINE BREEDING AND NARROW BREEDING. 

OTHER METHODS OF LINE BREEDING. 

Having once recognized that the vegetative propagation of our cul- 
tivated jDlants is a form of longevit}^, it becomes eas}^ to see that other 
methods of line breeding are very similar to vegetative propagation. 
The only difference between vegetative propagation and partheno- 
genesis is that in the latter the vegetative development of the new 
individual arises from tissues which usually serve the purpose of 
rei^roduction rather than from parts that are purely vegetative. 
The .cells which normally produce the germ cells grow into a small 
embryo-like bud, so that seeds are developed without access of pollen. 

Self-fertilization is not far from parthenogenesis, for it is accom- 
plished by reuniting cells of the same plant only recently separated. 
In self-fertilization the appearance of normal conjugation is main- 
tained, but the results do not correspond to those of the interbreeding 
of different individuals. Self-fertilized tj^pes show the sanie uni- 
formity of character expression as vegetative varieties, the same 
failure to permanently maintain the life of the stock, and the same 
gradual loss of vitality with age. 

In-and-in breeding is the nearest approach to self-fertilization pos- 
sible in plants or animals which have the individuals of different 
sexes. The germ cells which unite are from the most closely related 
individuals. 

In vegetative propagation and parthenogenesis there is no inter- 
rujDtion of the process of conjugation, for no new germ cells are 
formed. In self-fertilization and in-and-in breeding germ cells are 
formed, but the conditions of normal sexual reproduction are avoided, 
for cells of the same ancestry are brought together again, instead 
of cells of different ancestry. Though the formalities of sexual 
reproduction are repeated, there is in realit}^ no more sexualitv, in 
the physiological sense, than in vegetative propagation, for the 
unions of cells represent mere renewals of old conjugations, instead 
of being trul}^ normal new conjugations between partners represent- 
ing different lines of descent. 

Parthenogenesis is plainly intermediate between vegetative propa- 
gation and self-fertilization. Parthenogenetic and self -fertilized 
types can not be found, of course, in a state of complete seedlessness 
like some of our vegetative varieties, for seedlessness in a seed- 
propagated variety would mean immediate extinction. That so 
many vegetative varieties belonging to very diverse families have 
become seedless is one of many evidences that even tlie most success- 
ful form of line breeding does not maintain the full and normal 
vigor of organic types. The frequency of vegetative propagation 
and self-fertilization among cuhivated species has tended to give 

146 



SELF-FERTILITY. 15 

a misleading idea of the general importance of these methods of 
reproduction. 

A few species of plants, and especially certain degenerate parasites, 
appear to be uniformly parthenogenetic, but in the great majority 
of cases parthenogenesis appears as an alternative of normal fertili- 
zation, just as many species resort to self-fertilization or to in-and-in 
breeding when there are no opportunities of normal crossing. 

The domestication of i^erennial plants propagated by cuttings was 
verj^ much easier for primitive man than that of annual species of 
which seed had to be saved. Thus the earliest domestications were 
confined largely to perennial root crops and to trees, the annual species 
being relatively late acquisitions, as shown by the fact that most of 
them are still known in the wild state.** 

Many of our self- fertilized domesticated types have open-fertilized 
wild relatives, indicating that the habit of self-fertilization was not 
the primitive condition of descent. The same is true of most, if not 
all, of the self- fertilized wild plants; either they are occasionally 
crossed or they have cross-fertilizing relatives. Among the seed- 
propagated cultivated species the tendency of selection is generally 
toward self-fertilization, even where no such selection has been 
intended. 

Plants are often carried into regions Avhere their insect friends or 
other natural agents of cross-fertilization are absent or where the 
climatic conditions are unfavorable for the transfer of pollen through 
the air. The varieties of wheat and other cereals developed in north- 
ern Europe show more joronounced adaptations for self-fertilization 
than those of the Mediterranean countries. This can be explained 
by the selective action of the northern climate. Unfavorable weather 
at the time of flowering might caiise a total failure in a crop depend- 
ent upon cross-]3ollination and allow seed to be saved only from self- 
l^ollinated plants. In a similar way it is possible to explain the special- 
prevalence of vegetative proj^agation, parthenogenesis, and self-fertili- 
zation among plants that bloom early in the spring when unfavorable 
weather is likely to prevent cross-pollination by insects. 

RELATION OF SELF-FERTILITY TO VEGETATIVE PROPAGATION. 

Darwin found that some plants jdelded better progeny by self- 
fertilization than when the pollen came from other individuals of 
the same stock, and later experimenters have reported similar results 
in several self -fertilizing species. Some of the most notable instances 
are reported among the varieties of tobacco studied by Mr. A. D. 

" Cook, O. F. . The American Origin of Agriculture. Popular Science Monthly, 
October, 1902. Reprinted in the Annual Report of the Smithsonian Institution 
for 1903, p. 481, under the title " Food Plants of Ancient America." 
146 



16 LINE BREEDING AND NAEROW BREEDING. 

Shamel, of the Bureau of Plant Industry. Plants fertilized by their 
own pollen gave, in numerous instances, progeny much better on 
the average than plants fertilized by pollen from other individuals 
of the same stock. Individual members of the cross-fertilized series 
were equal to members of the self-fertilized series, but many were 
notably inferior as well as more diverse among themselves, which 
seriously detracts from the commercial value of the product. 

These experiments are of special interest because it was ascertained 
at the same time that hybrids between two distinct strains of tobacco 
showed less individual diversit}^ than when crosses were made among 
members of the same strain. Thus it becomes apparent that the 
diversity and relative inferiority of the progeny of the individual 
crosses is not to be ascribed to the crossing as such, but to the condi- 
tion of narrow breeding which such a cross represents, as compared 
with the line-bred stock on the one hand and with the broad-bred 
crosses between strains on the other. We may think of the inferiority 
shown in the narrow-bred stocks as a result of insufficient diversity 
of descent as compared with broad-bred stocks, and at the same 
time we may recognize that self-fertilization serves to postpone the 
degeneration by combining cells as closeh^ alike as possible. 

Thus the analogies of vegetative propagation help us to understand 
how two cells of the closest relationship may continue reproduction 
to better advantage than those of slighth" more distant relationship. 
Slight differences in the germ cells may be large enough to call forth 
individual diversity in the offspring and yet not large enough to give 
them the advantage of renewed vigor like that obtained by normal 
conjugation between germ cells derived from distinct lines of de- 
scent. The classing of self-fertilization with vegetative propagation 
is thus to be justified by the most practical reasons — that both attain 
the same results in producing a uniform progeny by eliminating the 
individual differences found among organisms produced by normal 
cross-fertilization. 

That self-fertilization and cross-fertilization yield different results, 
llie one of uniformity, the other of diversity, is a reason for believing 
that they constitute different processes of reproduction, though the 
nature of the difference is not yet known. The best suggestion of 
what this difference may be is afforded by a fact recently reported 
by Dr. Reginald R. Gates, of Chicago University, who has made a 
very detailed investigation of the processes of reproduction in a 
variety of evening primrose {Oenothera rithrinerris). The nuclei 
of the pollen mother cells were found to pass through a stage of con- 
traction (synapsis) and to divide into pollen cells Avith the usual 
reduced number of chromosomes, but without giving any indication 
that a fusion takes place between two parallel strands of chroiuatin 

146 



IN-AND-IN BREEDING. 17 

(mitapsis), the concluding act of the normal process of conjugation, 
<!escribed by many investigators of other plants and animals. Such 
a method of reproduction without a conjugation of chromatin 
(amitapsis) is very interesting as indicating that an apparently essen- 
tial part of the process of conjugation may be omitted though all 
the external formalities of conjugation are preserved. Xevertheless, 
amitapsis may be considered a less violent departure from normal 
sexual i)rocesses than is parthenogenesis. Amitapsis could be reck- 
oned as a production of sex cells by vegetative subdivision of a 
mother cell, parthenogenesis as the vegetative development of an 
undivided mother cell into a new organism. 

If these observations on Oenothera should be confirmed and ex- 
tended to other line-bred varieties we would have a reason for looking 
upon the individual diversity of cross-fertilized types as a result of 
the process of mitapsis, which appears to be omitted in these self- 
fertilized primroses." And even if it were found that the behavior of 
the chromatin has no relation to other characteristics of the plants, 
we should not forget that uniformity and diversity are concrete facts 
in need of physiological explanation. 

IN-AND-IlSr BREEDING. 

Not a few plants are like the higher animals in having the sexes 
represented by sej^arate individuals. In all such cases self-fertiliza- 
tion is of course an impossibility. The nearest approach to it is the 
mating of the most closely related individuals, sometimes called in- 
and-in breeding. Breeders of animals often secure better results by 
mating the most closely related individuals than by mating those 
that are a little less closely related, just as breeders of plants find 
the complete self-fertilization of a flower by pollen from its own 
stamens better than narrow crossing with pollen from other flowers 
of the same plant or from a closely related plant. 

In some varieties breeding with close consanguinity has been applied 
Math good results for a considerable series of generations. Other 
varieties have appeared to improve by in-and-in breeding, but soon 
show sterility or other degenerative weakness. In still other cases 
the reports indicate prompt and definite injury from in-and-in breed- 
ing. Relying on these individual experiences some breeders advocate 

^ " No indication of a doubling or pairing of tbe ttireads during these inter- 
mediate contraction stages could be observed, tliough they were carefully 
searched for. Moreover, in the earliest stages of the synaptic ball the thread 
appears to be as thin and delicate as in the reticulum, which does not favor 
the view that a pairing has taken place. The evidence, then, so far as it goes, 
is decidedly not in favor of a pairing." — R. R. Gates, A Study of Reduction in 
Oenothera Ruhrinervis, Botanical Gazette, vol. J/d, p. 8, July, 1908. 

76684— Bui. 146—09- 3 



18 LINE BREEDING AND NAREOW BREEDING. 

in-and-in breeding as a general principle of the art, while others 
condemn it as a wholly mistaken policy. 

A part of the diversity of opinion is due, no doubt, to inherent 
differences in the longevity of the stocks. A part may also arise from 
the fact that some breeders have been dealing with poultry or other 
small animals which are usually allowed to run in flocks instead of 
being separated into individual lines of descent. While a flock might 
promptly decline under narrow breeding, careful in-and-in breeding 
of select individual lines of the same stock might preserve superior 
strains. Yet this same policy applied in a different species or even 
in a different variety of the same species might only hasten disaster, 
if the necessary protoplasmic longevity w^ere lacking. 

There is even less probability that a type can be permanently main- 
tained by in-and-in breeding than by self-fertilization or by vege- 
tative propagation, but the period of endurance is undoubtedly long 
enough in some species to give the method practical importance 
where uniformity and the special development of particular charac- 
ters are more essential than to maintain the reproductive energy of 
the stock, the function in which the earliest decline may be expected. 
Thus it might be good policy to apply methods of strict self-fertiliza- 
tion or in-and-in breeding in localized varieties of cotton yielding 
special grades of fiber or in varieties of sheep producing a high-priced 
wool. The same systems might prove very unwise if the primary 
objects were to increase the total yield of cotton or to render the 
sheep more hardy and prolific. 

SELF-FERTILIZATIOJSr SUPPLEMENTS BROAD BREEDING IN 
NATURAL SPECIES. 

The flowers of some plants are so constructed that the pollen can 
readily fall upon the stigmas, wdiile in others the parts are arranged 
so as to prevent self-fertilization. This fact has often been used to 
support the idea of a natural duality of methods of reproduction, an 
arbitrary difference of reproductive methods among the diffVrent 
species, not to be reconciled and interpreted by the same physiological 
principles. The failure to distinguish properly between the three 
different types of reproduction — broad breeding, narrow breeding, 
and line breeding — has kept contradictory opinions alive in the same 
way as in the question between cross-breeding and inbreeding. 

The factor of longevity, the ability of the protoplasm to continue 
growth without a truly sexual reproduction, needs to be taken into 
account in understanding the great diversities of plants with respect 
to self-fertilization. Similar and nearly related types often differ 
widely in their ability to sustain themselves through self-fertilization, 

146 



BROAD BREEDING AND MNE BREEDING. 19 

just as other groups of species and varieties differ among themselves 
in their powers of vegetative propagation. 

If normal seeds can not be developed by self-fertilization, it is an 
advantage to a plant not to have its pollen fall on its own stigmas 
but to leave these unfertilized as long as possible, to increase the 
chances of arrival of the necessary foreign pollen. When good seeds 
can be formed by self-fertilization it becomes an advantage to the 
species to have its flowers so arranged that the stigmas are not left 
without pollen, just as it is an advantage to a plant able to propagate 
b}^ vegetative growth to have also the habit of forming offsets. 
Whenever this power of sustained vitality exists it is obviously ad- 
vantageous to the species to find ways of utilizing it. In all such 
cases self-fertilization is not to be thought of as a substitute for 
broad breeding, but rather as an equivalent of vegetative propagation. 

A plant may have the power of vegetative propagation residing in 
the protoplasm, but may lack the habits of growth necessary to turn 
this power to practical use. Thus gardeners find it possible to propa- 
gate from cuttings many plants wdiich gi'ow in nature only from 
seeds. The date and various other palms produce, when young, 
vegetative shoots from the buds of the lower joints, but mature palms 
no longer produce such shoots. The buds of the trunk bring forth 
only clusters of flowers. It often happens that of two closely re- 
lated species one will produce vegetative shoots and tlie other will 
not. Thus the Canary Island date palm produces no shoots, though 
it is a more hardy and vigorous species than the true date palm. 

BROAD BREEDING AND LINE BREEDING IN THE SAME SPECIES. 

That broad breeding and line breeding both have important uses 
in nature is shown by the fact that both are assisted by numerous 
and often highl}^ specialized adaptive characters. These specialized 
characters become still more significant when we reflect that there 
appear to be no corresponding adaptations to favor narrow breeding. 

Adaptations of plants to secure pollen from abroad and adapta- 
tions to insure the use of their own pollen at home both serve a 
common object in avoiding the worst alternative of narrow breeding. 
Indeed, the same plants often show both kinds of adaptations at 
once. The same species may have its flowers adapted to invite far- 
flying insects or pollen-carrying birds, and may have at the same 
time devices for excluding ants and other small visitors that can 
onl}^ crawl or fly for short distances and thus bring pollen only 
from flowers of the same plant or from adjacent plants, which are 
likely to be closely related. 

Many of these apparently contradictory combinations of adapt- 
ive characters have been found in nature. They have been used 

146 



20 LINE BREEDING AND NAEROW BREEDING. 

by several writers to show that Darwin's idea of the development of 
adaptations through natural selection was erroneous, on the ground 
that it is illogical to suppose that the same plant could at the same 
time develop adaptations for conflicting purposes. 

From the present point of view it becomes evident that there is 
no biological conflict, but an important physiological agreement 
between the functions of the two kinds of adaptations. They can 
be considered as alternative methods of avoiding the same catas- 
trophe of extinction through narrow breeding. Self-fertility can 
not be of use to a plant without some underlying quality of proto- 
plasmic longevity, like that shown in vegetative propagation, but 
when that equality exists self-fertilization may take on an impor- 
tance only second to normal broad breeding. It is not necessary to 
suppose that self-fertilization takes the place of broad breeding or 
that it serves all the functions df normal sexual reproduction. A 
safer judgment is reached by comparing self-fertilization with vege- 
tative propagation, since there is a large measure of agreement in 
the functions and limitations of the two processes. 

That plants with conspicuous or highly specialized corollas are 
nevertheless capable of self-fertilization does not prove that the 
corolla is not an adaptive development to aid in cross-fertilization. 
The specialization of the corolla only shows in a more striking man- 
ner that cross-fertilization is of importance to the plant, notwith- 
standing its ability to propagate by self-fertilization. That a species 
produces seeds with its own pollen or that it is able to survive many 
generations of self-fertilization does not prove that the species has 
ceased to draw any advantage from crossing, even though its oppor- 
tunities for crossing are relatively rare. The occasional exercise of 
normal sexuality may be quite as important for a self-fertilizing 
species as for one whose members are very long lived. Though self- 
fertilization is a frequent and entirely normal method of propagation 
for many plants, exclusive dependence upon self-fertilization is a 
very rare and abnormal condition, doubtless because it leads to ulti- 
mate decline and extinction, so that the types which may have adopted 
this system in the past have not been perpetuated. 

LINE BREEDING A SYSTEM OF PROPAGATION. 

Tn domesticated varieties, as in wild species, the chief value of 
self-fertilization and of other forms of line breeding lies in their use 
as methods of proj^agation rather than as true equivalents of sexual 
reproduction or as means of evolutionary i)rogress. Marked im- 
provements are as little to be expected from self-fertilization as 
from vegetative proi)agation. This does not mean that self-fertilized 
stocks are not worthy of the breeder's close and constant attention, for 

146 



PROPAGATION BY LTNE BREEDING. 21 

while there are not likely to be changes in the direction of greater 
vigor or increased fertility, tendencies in the other directions are 
frequently manifest. The practical importance of keeping a variety 
from deterioration may be quite as great as that of causing it to make 
an actual advance. Differences too slight to be detected in direct 
comparisons of individual plants of wheat maj?^ render one strain dis- 
tinctly more productive than another, and ma}^ make a difference of 
millions of bushels in the harvest when the superior strain has been 
spread over the wide regions in which the crop is cultivated. 

Unfortunatel3% this kind of selection, to increase or to maintain 
the vigor and fertility of a line-bred type, is often the most difficult 
to carry out in practice, one reason being that these same qualities are 
most seriously affected by external conditions. All breeders are 
familiar with the fact that conspicuously vigorous and fertile indi- 
viduals may yield only ordinary offspring. An individual cotton 
plant of a Mexican variety grown at San Antonio, Tex., in 1906, pro- 
duced much more cotton than any other plant in the field, but the 
offspring of this plant showed no superiority in the next generation. 

Wlien individual differences due to differences of conditions are 
greater than the inherent differences of the plants themselves, the 
inherent differences become very hard to detect. Even when the 
comparison is carried over to the progen}^ by using the test-row or 
centgener methods, the greatest caution has to be used to avoid mis- 
takes from unrecognized differences of external conditions. The 
manner in which a field was plowed or fertilized in some previous 
year may make a notable difference between two rows or plots, may 
cause the wrong selection to be made, and may thus vitiate an elab- 
orate experiment. The failure of such efforts at the selective im- 
provement of varieties in vigor and productiveness has led some 
writers to deny that such improvements are possible. 

Various attempts have been made to explain the supposed impossi- 
bility, some writers going so far as to claim that the characters of the 
separate lines of descent do not vary at all except as they are affected 
by differences of external conditions, and hence that all selection is 
superfluous after " pure lines *" of descent have been separated. The 
fact is, however, that many of the experiments that have been sup- 
posed to warrant this conclusion have been carried on for altogether 
too short a time to justify any reasonable expectation that the results 
of the selection would become apparent amidst the confusion of fluctu- 
ating and environmental variations. 

It is to be expected, of course, that attempts to secure a further 
increase of a particular character must sometimes fail if the limits 
of expression of the character have been reached or if the particular 
breed or line of descent has no tendency toward greater variation in 

146 



22 LINE BREEDING AND NAREOW BREEDING. 

that particular direction. But, on the other hand, we may expect 
with much confidence that a sufficiently persistent selection of vigor- 
ous and fertile individuals for propagating a variety will help to 
maintain these qualities at higher standards than if selection were 
relaxed and the stock were allowed to become diluted with lines of 
descent which had lost some of the varietal characteristics by de- 
generative mutation. Selection of this kind is safer and more likely 
to insure continuously favorable results when it is not too closely 
limited to single individuals or to single lines of descent, at least 
after the initial generation. Unless elaborate precautions are main- 
tained in the testing of each of the lines of descent, a serious mistake 
may at any time be made by propagating from an inferior individual 
which has appeared better than its fellows because of some environ- 
mental advantage. With a somewhat wider selection there is less 
danger of discarding the best lines, while those that are inferior are 
gradually weeded out. 

If selection could be finished, once for all, by the separation of a 
" pure line," any amount of care would be practically justified in mak- 
ing this separation, but if the superiority of a variety has not only to 
be made by selection, but also to be maintained in the same way, there 
is less object in narrowing the stock down to the individual line, 
particularly in the kinds of plants that must be sown or propagated 
for several gejierations to obtain enough seed for commercial planting. 
By the time that the single line has been broadened again to produce 
commercial quantities of seed, the differences may reappear, so that 
the type may be no more constant than if several individiuils had been 
taken and a smaller number of generations had been raised. Indeed, 
our experiments with cotton have shown that uniformity is to be 
expected with more confidence from a type represented by several 
closely similar plants than from the progeny of a single individual 
plant, which is often notably diverse. Thus it may happen, where 
strict line breeding is impracticable, that a choice must be made be- 
tween methods that most nearly approach the desired condition. 

If selection could compel actual improvements of a stock, vegetative 
and self-fertilized varieties could be advanced more effectively than 
any others, for selection can be applied with greater exactness and 
persistence than in cross-fertilized types. And yet it is in the former 
classes of varieties that the limitations of selection have been most 
definitely appreciated. 

Even among vegetative individuals of the same stock slight differ- 
ences are still to be detected, and sometimes quite pronounced changes 
or " bud mutations " are found. There is no reason to suppose that 
bud mutations differ in any essential respect from nuitations that 
arise in seedlings of narrow-bred or line-bred varieties. Nor do they 

146 



PROPAGATION BY LINE BREEDING. 23 

need to be thought of as being less sexual than the variations of seed- 
lings, now that we know that the sexual condition (conjugation) con- 
tinues throughout the existence of the plant even when this existence 
is prolonged by vegetative propagation. The only distinction that 
can be made with certainty is that bud mutations are generally much 
less frequent than seedling variations, there being, apparently, a 
greater facility of change of characters soon after conjugation be- 
gins. Breeders of potatoes believe, according to Dr. E. M. East, th^t 
bud mutations are much more frequent in new varieties recently de- 
rived from seedlings than in the same varieties after they have been 
grown for a long series of generations.'^ 

Whether the range of diversity in bud mutations is as wide as 
among seedling variations is not easy to determine, because the bud 
mutations are too rare to permit them to be studied in large numbers 
like seedling variations. Hundreds of seedling mutations of coffee 
have been observed by the Avriter in Central America, but only one 
bud mutation. And yet this showed its divergent characters in a 
manner quite as pronounced as any of the seedling variations. 
. The relative infrequency of bud mutations may also be responsible 
for the fact that they have had little practical importance. Breeders 
naturally feel that they have much better opportunity of selection 
among the rich diA^ersities that can be called forth by returning to 
sexual reproduction. Certain it is that very few valuable new types 
appear to have been derived from bud mutations — nothing to compare 
to the superior seedlings of apples, potatoes, sugar canes, strawberries, 
and many other species. Seldom, if ever, have bud mutations been 
found stronger or better than the parent stock, except from the stand- 
IDoint of the florist or fancier interested in multiplying slight differ- 
ences of form or color. 

We use vegetative propagation to preserve the varieties, but resort 
to sexual reproduction when we Avish to improve them, especially in 
the direction of vigor and fertility. The same vegetative variety may 
deteriorate faster under some conditions than under others, or it may 
even regain some of its vigor when conditions are improved. It has 
been found in Java that sugar canes brought down from elevated 
localities are more resistant to disease than other representatives of 
the same vegetative variety which have been grown continuously at 
low elevations. The change of conditions gives something of the 
same beneficial effects that are obtained by new conjugations. New 
seedling varieties of cane have also been found resistant to diseases 
in the same way as these mountain-grown vegetative strains. 

» East, E. M. A Study of the Factors Influencing the Improvement of the 
Potato. Bulletin No. 127, University of Illinois, Agricultural Experiment Sta- 
tion, August, 1908. 

146 



24 LINE BREEDING AND NAREOW BREEDING. 

HOW SELECTION IMPROVES LINE-BRED VARIETIES. 

The fact that selection increases the agricultural value of line- 
bred varieties can be understood without supposing that the plants 
are changed in any way bv the selection. The effect of selection is 
not biological, but purely mathematical. Selection does not give us 
any new characters, and does not even raise the vigor or fertility of 
the plants. It simply gives us the full agricultural use of the par- 
ticular lines of descent that are showing the vigor, fertility, or other 
desirable qualities in the highest degree. 

A notable improvement of a variety appears to have been made 
when the best individual strain has been found and the remainder of 
the variety has been discarded, but this is no warrant for holding 
that the superior strain is better than it was before. Selection then 
maintains the superiority of the strain by keeping it from dilution 
with the lines of descent in which degeneration appears, but still 
there is no reason to suppose that selection changes the strain. 

Though continued selection may add nothing new in the way of an 
increase of the desirable qualities of the varietj^, the same reasons for 
continuing the selection of the variety will always remain that ex- 
isted before the first selection was made, to eliminate the undesirable 
diversities which continue to appear even in self -fertilized and vege- 
tative varieties. Careful selection among individuals of such varieties 
may always be expected to show good results. In old and weak 
varieties the results will be even more striking than in the stronger 
and more vigorous stocks, for the greater the tendency to degenera- 
tion the greater the contrast will be between the degenerate lines of 
descent and those that still retain the characters which give the 
variety its special value. 

SELECTION A CONSERVATIVE PROCESS. 

Favorable results from individual selections in a narrow-bred or 
line-bred variety is no proof that the variety as a whole is advancing 
in excellence, but may indicate quite the contrary — that its vigor is 
declining. The more rapid the decline the larger is the j^roportion 
of degenerate individuals and the greater the practical improvement 
worked by selection. Tlie greater need of selection in degenerating 
varieties is like the reciuirement of more efficient police in degenerate 
human coiuniunities. The reinoMil of the criminals is a practical 
necessity, though it does not make the comnumity better, except in 
the rehitive sense that it may lessen the amount of crime actually 
connnitted. In the same way it is necessary to maintain a careful 
supervision of chjse-bred varieties to keep them near the standard of 
eiliciency by eliminating the degenerate lines of descent as soon as they 
fall l)el()w the standard. 

140 



SELECTION A CONSEEVATIVE PROCESS. 25 

To compare the best lines of descent with those that have degener- 
ated or with an unselected group containing degenerate lines shows 
the agricultural value of the selection, but it does not show that the 
best lines have been made better than they were. Domesticated varie- 
ties can be improved agriculturally without being changed biologic- 
ally. There is no reason to suppose that the sorting out of the 
degenerate lines of descent in a strictly line-bred variety has any effect 
upon the lines that are retained. The practical advantage of main- 
taining selection can be understood without any need of supposing 
that the best lines are becoming better than they were. If a tendencj^ 
to degeneration, such as seedlessness, paler color, or softer texture, 
renders a variety more desirable, the lines which are degenerate, in 
the biological sense, will be preferred to the others and the variety 
will appear to be the more improved agriculturally the faster it 
degenerates. It may be going too far to say that all the instances 
Avhere persistent selection has resulted in continued improvement in 
economic characters are examples of biological degeneration, but it 
is certain that a verj'^ large majority are of this nature. 

We need to recognize that there are two separate branches of the 
art of breeding, constructive breeding and conservative breeding, 
having different objects and requiring different methods. The former 
attempts to improve the characters of plants and animals ; the latter, 
to preserve and make full use of desirable characters already obtained. 

Some would restrict the word breeding to the constructive idea of 
improvement, which comports with the suggestion that all the proc- 
esses of line breeding are to be considered as methods of propagation 
rather than as normal breeding in the sense of sexual reproduction. 
Nevertheless, such a limited use of the word breeding would exclude 
a large part of what is now reckoned as breeding, if not, indeed, the 
whole of the practical part of the subject. Breeders are often very 
successful in finding new characters not noticed before, as well as in 
making new combinations of desirable characters and in suppressing 
undesirable characters, but there is relatively little to support the 
popular idea that the operations of breeding result in " new creations," 
in the sense of bringing new characters into existence outside of those 
already attained in the course of normal evolution of species. 

Whatever the differences of opinion regarding the function of 
selection in constructive breeding, its fundamental importance in con- 
servative breeding will not be disputed. The superiority of the vari- 
ous forms of line breeding over narrow breeding is based on the 
simple fact that they preserve desirable characters more effectively 
and for longer periods of time than does narrow breeding. Line 
breeding is superior to narrow breeding because it is a more efficient 
method of selection. 

146 



26 LINE BEEEDING AND NAEKOW BREEDING. 

The beneficial effects of selection are greatest in self-fertilized 
varieties because such varieties are no longer in a condition in which 
the}^ can be injured by selection. A broad-bred variety may be in- 
jured or extinguished by selection, because strict and persistent selec- 
tion puts an end to broad breeding and substitutes narrow breeding 
in its place. A variety capable of self-fertilization is less injured by 
narrow breeding, for those lines of descent which continue to be 
propagated by self-fertilization also continue to escape the bad 
effects of narrow breeding. Thus the tendency of persistent selection 
will be to preserve those strains in which the habit of self-fertiliza- 
tion is most strongly developed. When a condition of complete self- 
fertilization can be reached., as in wheat and barley, there is no 
longer any question of injury from narrow breeding, since all the 
lines of descent are propagated by line breeding. 

Selection between lines of descent which are already completely 
separated by self-fertilization makes no further alteration in their 
conditions or methods of reproduction. Selection then has only the 
one effect of preserving the lines which express in the most uniform 
manner the characters most desired. If we take the trouble to choose 
the best strain we are assured of the best results. In no other way 
could this assurance be gained, for by any less stringent method of 
selection there is always the danger that representatives of poor or 
mediocre stocks ^re included and that the full possibilities of selec- 
tion are not attained. 

But no matter how effective the methods of line breeding may be 
in varieties adapted to these forms of i^ropagation, their general in- 
discriminate application to all kinds of plants is not to be advised, 
notwithstanding that this course is very frequently advocated, as 
a consequence of the evolutionary doctrines of Professor De Vries. 
Rejecting Darwin's doctrine of the evolution of species through 
gradual changes of characters, De Vries holds that species originate 
by sudden changes or mutations, like those which appear in our 
uniform domesticated varieties. Ordinary species and varieties are 
supposed to consist of mixtures of large numbers of these so-called 
'* elementary species," or " biotypes," originated by mutation. AVhen 
one of these mutative strains has been separated by selection it is 
sujiposed to remain constant and uniform, except as further muta- 
tions of individuals may give rise to additional '" species.'' 

This doctrine has had in many instances the very beneficial effect of 
directing the attention of breeders to the advantage of line breeding, 
l)nt it brings us into conflict with practical facts when it leads us to 
assume that line breeding is applicable to all kinds of organisms, and 
also when it teaches that no further .selection is necessary after a 
" puie line" has been separated. No better illustnition of pertinent 

146 



UNIFOBMITY NOT NORMAL. 27 

facts could be found than the Triumph variety of cotton, originated 
by Mr. Alexander Mebane, of Lockhart, Tex., and carefully selected 
by him for many years. Mr. Mebane's fields of Triumph cotton at 
Lockhart show wonderful uniformity, as all cotton specialists have 
agreed who have seen them. And yet when the Triumph cotton is 
taken to other places, even at no very great distance from Lockhart, 
many notable deviations from the Triumph characteristics make their 
appearance. This occurs even in the first generation of plants grown 
from seed raised at Lockhart, before there is any opportunity for 
crossing with other varieties or for mutations to take place as the 
result of conjugation subsequent to the transfer. The Triumph 
embryos in the seeds or the young plants after they have germinated 
are able to change their characters and depart from the Triumph 
type. These departures, moreover, are commonly inherited by alj 
the progeny of the variant individuals. The persistence of such 
variations soon puts an end to the uniformity of a variety unless 8 
vigilant selection be maintained. 

UNIFORMITY NOT A NORMAL CONDITION OF HEREDITY. 

That the effects of selection upon self-fertilized varieties are 
always agriculturally beneficial makes it possible to understand the 
growth of the idea that uniformity is a fundamental principle of 
heredity and the first ideal of the breeder's art. Uniformity has 
come to be considered as an object in itself, for the very practical 
reason that selection for uniformity tends to carry self-fertilized 
varieties over from narrow breeding to line breeding even when 
the breeder has had no intention to make such a ch.ange of method. 
To adopt as a fixed ideal of selection the characteristics of the best 
individual member of the variety generally amounts in practice to 
the saving of the progeny and nearest relatives of the superior indi- 
vidual. The more strict the adherence to the ideal, the greater the 
approximation to line breeding and the better the results. 

Nevertheless, it must be admitted that the value of uniformity as 
a method of breeding one class of varieties does not establish it as 
a general principle of heredit}^, to be applied to all varieties alike. 
Nor does the behavior of self-fertilized domesticated varieties show 
that uniformity is the natural condition of species in nature. Closer 
examination of the facts enables us to see why uniformity is desir- 
able in agricultural varieties, and also why selection for uniformity 
tends to produce the desired result. We do not find that the idea 
of uniformity applies to wild species nor in the human family, nor 
even in the dogs and other domestic species which we learn to know 
as individuals. Nor have we any assurance that uniformity is a 
safe general guide in the field of agricultural breeding. Even in 

146 



28 LINE BREEDING AND NARROW BREEDING. 

dealing with line-bred t3^pes we are no longer satisfied with likeness 
between individuals, but have learned to separate them into lines 
of descent and to judge of the value of a line of descent by the 
general quality of successive generations of progeny instead of from 
single individuals. In broad-bred types uniformity is obviously not 
the ideal, except to the extent that it may be necessary for commer- 
cial and industrial purposes. 

TWO FORMS OF MASS SELECTION. 

Writers on evolution have drawn distinctions between several dif- 
ferent forms or conditions of selection. The selection which goes on 
in nature, without human interference, Darwin called " natural selec- 
tion." Of selection by man Darwin distinguished two forms, 
unconscious selection and methodical selection. Selection becomes 
conscious when it has for its object the improvement of the stock. 
The selections carried on by primitive peoples are usually quite as un- 
conscious as those made bj^ birds, insects, or inanimate conditions of 
the environment. Thus, the Indians of eastern Guatemala have a 
very early, quick-maturing type of cotton, the development of which 
has been assisted, no doubt, by the fact that they begin tcr gather the 
cotton as soon as the bolls begin to open. This has insured the saving 
of seed from the earliest plants, not because of any idea of improving 
the crop in earliness, but because the dry season of that part of the 
country is short and uncertain. If the rains come too early, only 
the first pickings are saved; late-maturing plants are completely ex- 
cluded from the stock. 

The practical difference between this unconscious selection for 
earliness and the conscious selection practiced in more intelligent 
communities lies only in the keeping of the earliest pickings separate 
from the others. Among the Indians all the pickings are likelj^ to be 
mixed together in favorable seasons, so that a definite selection takes 
place only in unfavorable years. 

Later writers go beyond Darwin and divide methodical selection 
again into two forms, commonly called '" mass selection "" and " indi- 
vidual selection." This is because the progeny of single individuals 
kept by themselves often maintain higher averages than the progeny 
of several superior individuals of the same strain when allowed to 
interl)roed. From such facts the inforeuce has been drawn that mass 
selection is essentially inferior to individual selection and should 
everywhere be abandoned. Some writers even go so far as to deny 
thai an}' improvement can be wrought by mass selection, while others 
object that it is slow and inefficient. Nevertheless, practical breeders 
are loath to abandon as worthless the method by w hich all the earlier 

14G 



THE " EUNNING OUT " OF VARIETIES. 29 

improvements of our seed-propagated plants were made — the im- 
provements which have carried them so far beyond their wild relatives 
in the special qualities and which render them useful to mankind. 

In reality, two essentially different conditions are being confused 
under the name ' mass selection, the conditions which have been 
distinguished as broad breeding and narrow breeding. ^A-lien mass 
selection is applied to a carefully selected variety or strain, derived, 
perhaps, from a single ancestor, the condition is the same as that 
here described as narrow breeding. When mass selection is applied 
to larger groups with more normally diverse ancestry, we have a 
condition of consciously directed broad breeding, a form of mass 
selection which may be highly beneficial. 

The objections commonly urged against mass selection apply to the 
former condition, but not to the latter. JNIass selection in a self- 
fertilized plant like wheat is inferior to individual selection, or line 
breeding, simply because it is less thorough, as already explained in 
previous chapters. Mass selection gives us a collection of the better 
of the individual strains, wdiich must always be found inferior to the 
best of the strains whenever it is practicable'to separate these from the 
others. In cross-fertilized plants and animals mass selection may be 
inferior for the further reason that the narrow individual crosses 
arouse undesirable degenerative variation. 

In its application to self-fertilized plants a persistent individual 
selection constitutes line breeding. Nevertheless, it is dangerous to 
prescribe individual selection as a solution for all the problems of 
breeding, for it solves problems of only one kind and gives wrong- 
indications in the others. The theory of individual selection neglects 
the practical distinctions between the different forms of line breeding 
as well as the differences between narrow breeding and broad breed- 
ing. The expression "" individual selection " is in itself rather mis- 
leading, for all the methods of selection involve the selection of indi- 
viduals. The difference of methods does not lie so much in the selec- 
tion of individuals as in the breeding they receive after they have 
been selected. 

THE " RUNNING OUT " OF VARIETIES. 

Some horticulturists believe in the " running out " of varieties of 
apples, pears, and other tree fruits propagated from grafts or cut- 
tings, while others have been unwilling to admit that varieties run 
out. Such differences of opinion are possible in dealing with the 
varieties of long-lived trees, for our varieties have not been known far 
beyond the span of life possible for individual trees under favorable 
conditions. With potatoes, strawberries, lilies, carnations, and sugar 
cane the weakening of old varieties is a commonly recognized fact. 
Specialists in potatoes and strawberries estimate the productive life 

146 



30 LINE BREEDING AND NAREOW BREEDING. 

of a variety between twenty and sixty years. It is the opinion of 
Dr. B. T. Galloway that the frequent substitution of varieties among 
growers of carnations is due as much or more to the greater vigor of 
the new seedlings than to any superior attractiveness of the flowers. 
The commercial life of a variety of carnations seldom lasts more 
than three or four years. 

As vegetative varieties of many different kinds have shown the 
same tendency to become sterile, so it is possible that carefully selected 
types may reach a condition where any admixture of blood brings 
deterioration, as breeders sometimes claim. Improvement by crossing 
would then be precluded. The particular character which affords our 
standard of the excellence of a variety may appear for a time to 
increase under selection, but its highest expression is finally reached. 
Sterility or other forms of weakness become more and more apparent. 
New types arise to claim the superiority, and the old are soon aban- 
doned and forgotten. To know that a method of breeding leads 
ultimately to degeneration may not remove it from the field of agri- 
cultural utility if there are compensating advantages. It may be 
easier to secure new varieties to replace the old ones as they decline 
than to apply any effective methods for avoiding the decline. 

Those who are concerned to prevent the extinction of species of 
wild animals and plants may find an object of even greater practical 
interest in the danger of exterminating the wild types of our domes- 
ticated species, which future generations may find indispensable for 
replenishing the stock of domesticated varieties. 

Though the limits can not be determined without making the ex- 
periment, we must not expect that selection or any other means can 
lead to an unlimited expansion of any character. Cotton plants can 
not be turned conipletely into lint, nor beets made to yield 100 per 
cent of sugar. Characters should not be thought of as independent 
entities capable of indefinite expansion ; in reality, characters are 
only parts or functions of highly coordinated complex organisms. 
The existence of each character represents a cooperative result of the 
activities of other parts, so that the whole organism has to be unbal- 
anced in order to permit an excessive expression of a particular part. 

REJUVENESCENCE OF VARIETIES. 

It is not to be assumed that all of our carefully selected varieties 
have reached a condition where crossing can Avork no improvement. 
For many of them it may still be of the greatest importance to pro- 
vide normal fertilization at needed intervals. 

The object of such crosses is not that of changing the characters of 
the stock, but to restore the vigor. Some breedei*s have reported ex- 
cellent results from the rejuvenescence of varieties by crossing, but 

146 



BALANCED BREEDING. 31 

others have drawn back from such attempts because they encountered 
an unwelcome amount of variation. That diversity should appear 
as the result of crosses between varieties is of course to be expected, 
but it may be possible to eliminate this diversity in a few generations 
and yet retain an increased vigor for many generations. 

In normal species, as we have seen, line breeding is not a full 
equivalent of sexual reproduction, but a supplementary system of 
propagation. Nature shows that broad breeding and line breeding 
may be combined, but shows also that they should not be confused 
or compromised into the injurious intermediate condition of narrow 
breeding. In domestication, too, it may often be found advantageous 
to combine the two methods, without confusing them. We do not 
insist upon growing all potatoes from seed because new seedling 
varieties are often superior to the older stocks which have been sub- 
jected to many years of vegetative propagation. In the same way 
we may find it as advantageous to resort to occasional crosses in seed- 
propagated plants as to adopt newer and stronger seedling varieties 
in species adapted to vegetative propagation. 

Crossing between sufficiently remote strains of a line-bred variety 
may also be found highly advantageous and may help to maintain 
vigor, notwithstanding the fact that crosses between more nearly 
related lines of descent may yield results inferior to those of strict 
line breeding. 

Even without crossing it is sometimes possible to secure a notable 
increase of vigor or other desirable quality through the discovery of 
an unusual individual. Mutations often show the same sudden in- 
crease of vigor as hybrids, and this may continue in their descendants 
for many generations. Many important varieties of plants and 
breeds of animals trace their origin and popularity to a single superior 
individual. Though we can hardly say that we rejuvenate an old 
breed when we replace it wuth a new one, the same agricultural pur- 
pose is attained in the two cases, especially if the agricultural qualities 
are the same. 

A BALANCE BETWEEN BROAD BREEDING AND NARROW 

BREEDING. 

A system which has been followed w4th apparent advantage in 
some of the carefully bred varieties of domestic animals is that of 
wide and continuous crossing inside the breed. Instead of limiting 
descent to a single individual strain or to a few related strains, the 
effort is made to breed together individuals which show all the 
<iharacters of the breed and yet have as little consanguinity as possi- 
ble. Such a system, if efficiently carried out, as among the Australian 

146 



32 LINE BREEDING AND NARROW BREEDING. 

sheep breeders, keeps the lines of descent of the variety connected in 
a network. Though the network is narrower than in a natural species 
it is at the same time much broader than in ordinary narrow breeding. 
Such a condition might be described as the narrowest form of 
broad breeding or as the broadest form of narrow breeding; it 
represents the boundary or balance between the two methods. It is 
the condition in which a considerable degree of uniformity is main- 
tained and deterioration from narrow breeding is avoided as far as 
possible. For convenience of expression it seems best to consider 
this condition as one side of the field of broad breeding, because use 
is being made of crossing between different lines of descent to main- 
tain the vigor of the stock. The practical point is to avoid the con- 
dition of still narrower breeding betAveen this balanced condition in 
which the physiological effects of crossing are still utilized and the 
condition of definite line breeding in which narrow crossing is ex- 
cluded and reliance is placed on the other principle of protoplasmic 
longevity. 

AVOIDANCE OF UNNECESSARY UNIFORMITY. 

HoAv to maintain fertility by broader breeding and at the same 
time avoid an undesirable diversity of characters is one of the practi- 
cal problems which each breeder must consider from" the standpoint 
of the types with which he deals. By proper attention to the ex- 
pression relations of the different characters it may be possible to 
secure diversity of descent with relative^ little diversity of expres- 
sion or to confine diversities of expression to characters which will 
not interfere with the utility of the breed. 

In the cotton plant, for example, many characters might be allowed 
to vary freely without interfering in the least with the value of the 
product, and such variations would have positive agricultural value 
if they gave increased vigor and fertility. If the plants produce 
early crops of large bolls with lint of uniform length and fineness it 
makes no difference that stems have different colors or different 
amounts of hairs, that the leaves are differently cut, the flowers dif- 
ferently spotted, the nectaries and bractlets differently shaped, or 
the oil glands differently distributed. 

This question of the amount of uniformity to be required is of 
primary importance in the domestication or acclimatization of new 
types of plants, and especially of field crops that are subject to cross- 
fertilization, like cotton and corn, Avhere strict line breeding is not 
])rarti('able. Not to require unnecessary unifoi-mity in such cases 
docs not moan that selection can be safely relaxed in types that have 
ahvady been brought to a condition of uniformity, nor doe>; it justify 

14G 



UNNECESSARY UNIFORMITY. 33 

US in overlooking the fact that characters of no vahie in themselves 
may be worthy of careful attention when they are correlated with 
more iiriportant features. With cotton, for example, there seems to 
be a very definite correlation between the external form of the boll 
and the length of the lint. Plants that have longer and more pointed 
bolls are almost always found to have longer lint than neighboring 
individuals with more rounded bolls. It is therefore worth while 
to pay attention to the shape of the boll as well as to its size if the 
desirable qualitj' of long lint is to be carefully guarded. 

Similar correlations between unimportant characters and those 
that have definite utility have been detected by Mr. W. W. Tracy. 
sr., in varieties of corn, beans, lettuce, and other plants. In the 
Extra Early Adams variety, corn plants with branched tassels, even 
though they may be as early themselves as other plants with the un- 
branched tassels characteristic of this . variety, have been found to 
yield offspring with a distinct tendency to later maturitj'. Plants of 
the Golden Wax beans with flowers slightly larger than normal were 
found to yield offspring with a distinct tendency to j^roduce ordinary 
green pods instead of yellow. Certain varieties of lettuce which 
differ in the characters of the cotyledons have also been found to 
differ in the time of running to seed, even when they appear to be 
indistinguishably alike in the interval between. 

The corn plant has been the object of much selection, but this has 
been directed almost exclusively to the characters of the fruit instead 
of to the plant as a whole. The result is that even in varieties that 
give relatively uniform ears and kernels the vegetative characters 
continue to show wide ranges of diversity in form, color, and hairi- 
ness, as well as in the tassels and flowers. 

In a broad-bred stock it is evidently possible for many such fea- 
tures to continue their natural diversities without detriment. It is 
only in narrow-bred types that absolute uniformity has appeared a 
desirability of breeding, and this we can now undej'stand, because 
the application of such standards has always tended to carry these 
types toward the superior condition of line breeding. The efforts 
that have been made to place cross-fertilized types, like Indian corn, 
on the same basis of uniformity as a line-bred type, like wheat, have 
not had any corresjDonding measures of success. In spite of long and 
persistent selection only a small proportion of the plants show any 
complete uniformity of the fruiting characters. The facts with ref- 
erence to corn have been summarized by Mr. A. D. Shamel of this 
Department. 

It is tlie experience of most corn breeders that it is not possible to produce 
on an acre more than 5 busliels of luiiform ears even of onr most improved 
strains. A large majority of the plants pi'odiice ears of small size, irregular 
146 



34 LINE BREEDING AND NARROW BREEDING. 

sliape. and light weight, whicli are undesirable. Many of the stalks are barren. 
Only a small proportion of the plants produce the maximum size and weight of 
ear.'* 

Such facts make it evident that there is a very practical distinction 
between the cross-bred and the self-fertilized types with reference to 
uniformity. Selection toward uniformity increases the jdeld of self- 
fertilized types, because their methods of reproduction are well 
adapted to give regularity in the expression of the desired characters, 
whereas the cross-fertilized types have not the same power of pro- 
ducing uniform offspring. With the cross-fertilized plant there is 
no such direct and necessary connection between uniformity in high 
yield and uniformity in other characteristics. If corn could be 
propagated by self-fertilization it might be expected to respond to 
selection in the same way as the self-fertilized cereals, but no self- 
fertilized strains of corn have been developed. In the great majority 
of corn varieties self-fertilization is definitely prevented by the habit 
of proterandry, the ripening of the pollen of the plant before its 
silks are ready for fertilization. In an experiment with many varie- 
ties at Lanham, Md., in the season of 1908, a single variety proved 
to be an exception to this rule. The stalks produced silks and pollen 
at the same time, but the variety can hardly be said to be in a normal 
physiological condition, for the leaves of all the plants were thickly 
spotted with j^ellow, a sjanptom not shown in any other variety. 

Several experimenters have reported distinctly unfavorable results 
from self-fertilization in corn, the self- fertilized progeny being 
notably inferior to the cross-fertilized. It is possible that strains of 
corn may be found which will thrive under self-fertilization, but this 
is rendered somewhat less likely by the fact that the stamens and 
pistils are produced on different parts of the plant, instead of close 
together. 

The internodes of the higher plants have, as botanists know, a 
certain individuality of their own, the plant being a complex or 
colony of many of these internode individuals. If we consider the 
plants from this standpoint of the individuality of the internodes it 
becomes plain that the pistil and stamen internodes are verj' close 
relatives in the self- fertilized cereals, while in the corn they are sepa- 
rated by many generations of internodes, so that their relationship 
is relatively remote. The self-fertilization of a corn plant does not 
mean the uniting of cells from adjacent internodes, as on the wheat 
l^lant, but unites cells which are in some senses as little related to each 
other as though they were on diffierent plants. 

a Shnnu'l. A. D. 'I'lic Aii of Seed Si-lccl ion :ni(l I '.reed in. tf. Yoarhook of tlio 
I>(»l)ar(nu'ii( of .\i:rirni(nr«' foi- l!)(iT. p. T21. 

140 



THE '^ FIXING OF CHAEACTEKS " BY LINE BREEDING. 85 

Thus it appears that the specialized habit of corn to produce its 
pistils and stamens on different parts of the plant puts it out of the 
reach of the kind of self-fertilization practiced in wheat. In order 
to make corn experiments which shall be truly parallel with those of 
the self-fertilized cereals, it will be necessary to find varieties with 
hermaphrodite flowers, indications of which are sometimes found on 
abnormal suckers of our ordinary varieties. 

In view of these unusual 'obstacles to line breeding it is fortunate 
that corn has relatively little practical need of the uniformity which 
is of so much importance in crops like cotton and tobacco. The chief 
object in a corn crop is a large yield. A large proportion of the corn 
is not sold at all, but is used on the farms where it is raised. Farmers 
would be very willing to permit variations, even in the shapes and 
colors of the grains, if they could get enough more corn per acre. 

Experiments have shown that increased jdelds can be obtained by 
crossing varieties of corn. Indeed, the fact seems to have been 
utilized since prehistoric times among the Indians of the Quezalte- 
nango region of western Guatemala. Thej?^ follow the practice of 
planting three different types together for the bulk of their crop, in 
the belief that larger yields are obtained in this Avay than when the 
varieties are planted separately. Nevertheless, breeders remain loath 
to advise mixed plantings of corn, or even to investigate such a possi- 
bility of increasing the yield, perhaps because this advice is so far 
out of accord with the methods by which some other cereal crops have 
been improved. 

THE " FIXING OP CHARACTERS " BY LINE BREEDING. 

Recognition of the superiority of line breeding over narrow breed- 
ing may help to explain the belief of many breeders that inbreeding 
" fixes " characters. Characters may appear to be fixed because line 
breeding protects a selected stock from the degenerative diversity 
which comes with narrow breeding. W^ien such a line-bred stock is 
crossed with a narrow-bred variety there may be a definite tend- 
ency for the peculiarities of the superior stock to predominate in 
the offspring. 

The mutations of narrow-bred and self- fertilized plants show that 
mere repetition does not insure stability, but may be followed by 
wide diversity, even after uniformity has been maintained for many 
generations. Selection has been supposed to eliminate undesirable 
characters, but in reality it is only able to postpone changes of ex- 
pression relations. The characters which the breeder would eliminate 
continue to be transmitted and may continue to reappear, even in 
the face of persistent inbreeding. 

146 



36 LINE BREEDING AND NARROW BREEDING. 

RECURRENCE OF DIVERSITY IN CROSSES BETWEEN LINE-BRED 

GROUPS. 

Further evidence that self-fertilization and in-and-in breeding are 
to be associated with vegetative propagation rather than with normal 
conjugation is found in the fact that diversity of expression reap- 
pears when diversity of descent is permitted to members of line-bred 
varieties. 

Normal broad breeding is accompanied by alternative expressions 
of characters involving continual readjustments of the internal re- 
lations which govern expression. Line breeding, on the other hand, 
avoids such readjustments. With respect to the expression of its 
characters a whole line-bred variety corresponds to a single individual 
of a broad-bred group. Diiferences between line-bred varieties cor- 
respond to differences between the individual members of species, 
not to differences between the species themselves. All the forms of 
line breeding yield a relatively great uniformity of characters, but 
there are suggestive differences in the readiness with which the latent 
diversity reappears. 

The uniformit}^ secured by vegetative propagation conceals but 
does not diminish the inherent diversity of expression relations. The 
individual diversity of seedlings of vegetative varieties is familiar 
to all propagators of such plants. Self-fertilized types may also 
show a ready recurrence of diversity of characters when sufficient 
diversity of descent is supplied to correspond to that of normal in- 
terbreeding among the diverse members of a species. In strictly self- 
fertilized types, like wheat, crossing between different individuals of 
the same variety may bring out diversities as wide as those shown 
by crosses between different varieties. 

In plants having a normal aptitude for self-fertilization the uni- 
formity of varieties is to be thought of as due to this fact of 
self-fertilization rather than to selection alone. In a strictly self- 
fertilized tj'^pe, selection can have reference onl^^ to the efficiency with 
which the characters are maintained under self-fertilization. It ap- 
pears to have no effect at all upon what may happen when crosses 
are made, even between individuals of the same variety. The 
adjustment which suffices for uniformity under self-fertilization 
would not be sufficient to maintain uniformity under in-and-in 
breeding. 

The effect of persistent selection is to conserve those lines of descent 
which have great uniformity of expressing relations along desired 
lines, for the others are rejected whenever their expression deviates 
in any aj^preciablo manner from the standard set by the breeder. 
Hence the desirability of definitely directing our selection toAvard 
uniformity of expression, as in the centgener method, which tests the 

146 



CONCLUSIONS. 37 

different lines of descent with reference to the quality of uniform 
excellence of the progeny rather than with reference to exceptional 
attainment on the part of particular individuals among the progeny. 

In types not definitely restricted to self-fertilization selection for 
uniformity has a more definite influence upon the results of conjuga- 
tion. With such types diversity comes less readily to the surface, 
even with crossing, but is still able to show itself in striking forms 
of mutative variation. Hybrids between two related varieties of 
Upland cotton commonly show a much smaller range of diversity 
than the mutations which appear in the pure-bred parental stocks. 
The diversity between varieties is not great enough to overcome the 
effects of selection and arouse the latent diversities which appear in 
the mutations, but when wider crosses are made between unrelated 
stocks the hybrids offer a wide range of diversities, corresponding 
to the diversities among the mutations. 

CONCLUSIONS. 

Long-standing differences of opinion among breeders regarding 
the values of crossing and inbreeding can be reconciled by recogniz- 
ing the fact that there are three primary conditions or methods of 
reproduction, instead of two. The three conditions may be called 
broad breeding, narrow breeding, and line breeding. 

In broad breeding there is no restriction of descent to particular 
lines. Unions are freely made among large numbers of lines of 
descent, as in natural species, where lines of descent are united into 
broad, continuous networks. In narrow breeding descent is restricted 
to unions among only a few lines, forming a narrow network of 
descent. In line breeding descent is restricted to simple lines, so 
that no network of descent is formed. 

Line breeding is superior to narrow breeding, but can not be con- 
sidered superior to broad breeding except for special purposes of 
commercial production requiring a high degree of uniformity. 
Broad breeding is the condition of normal evolution of species ; nar- 
row breeding the condition in which degeneration most promptly 
takes place; and line breeding the more stable and uniform condi- 
tion desired in many domesticated animals and plants. Broad 
breeding is constructive, narrow breeding destructive, and line 
breeding conservative. 

The uniformity of a group is increased by restricting descent so 
that all the individuals are' produced from few ancestors instead of 
from many. Descent from many diverse ancestors maintains the 
individual diversity of natural species. Diversity lessens as descent 
is restricted, but there is also a gradual decline in the vigor and 

146 



38 LINE BEEEDING AND NAREOW BREEDING. 

fertility of the stock. The practical importance of uniformity should 
not cause us to overlook the fact that uniformity must be attained 
at the price of deterioration. We must either avoid the deteriora- 
tion of varieties or replace them frequently with new varieties. 

Though all forms of restricted descent lead ultimately to degenera- 
tion, the decline may be exceedingly slow and gradual if methods of 
line breeding are followed. Line breeding has practical superiority 
over narrow breeding when it preserves desirable strains of plants 
or animals for longer periods of time. The ability of some species 
and varieties to maintain themselves under line breeding is to be 
considered as a form of longevity, depending on the power of the 
protoplasm to continue its activity without new conjugations be-. 
tween germ cells from different lines of descent. Species and 
varieties differ in their ability to persist under restricted descent, 
just as they differ in the longevity of the individual organisms. 

Four forms of line breeding may be distinguished: Vegetative 
propagation, parthenogenesis, self-fertilization, and in-and-in breed- 
ing. All stages and gradations can be found, from the broad breed- 
ing of natural species, through the narrow breeding of ordinary 
domesticated varieties, to the strict line breeding of individual 
strains. The superiority of broad breeding over narrow breeding 
depends on the factor of normal conjugation between cells derived 
from different lines of descent, whereas conjugations between cells 
of too closely related lines often produce weak or abnormal offspring. 

If conjugations are to take place between unlike germ cells, a con- 
siderable degree of diA^ersit}'^ of parentage is to be maintained. If 
conjugations are to be limited to closely related germ cells, diversity 
of parentage is to be avoided. In broad breeding we imitate the 
methods of descent in natural sexual species. In line breeding we 
follow the analogy of vegetative propagation and self-fertilization. 
A truer idea of the value of other methods of line breeding is gained 
when we associate them with vegetative propagation than when we 
consider them as equivalents of the sexual reproduction of broad 
breeding. That no group of higher plants or animals relies for its 
reproduction upon any form of restricted descent forbids the assump- 
tion that domesticated varieties can be permanently maintained 
under conditions of restricted descent. 

The superiority of vegetative propagation shows that complete 
cessation of conjugation preserves varieties better than conjugation 
without diversity of descent. In parthenogenesis conjugation is 
likewise in abeyance, tlie vegetative growth of a new organism taking 
the place of the formation of new sex cells. Self-fertilization is not 
far removed from parthenogenesis, for the cells that unite have only 

146 



CONCLUSIONS. 39 

recently separated from the same parent and are able to comliine 
without disturbins: the expression relations of the jDarental char- 
acters. 

Evidence of the superiority of line breeding over narrow breeding 
is also found in nature in the adaptations by which many wild 
species avoid narrow breeding. Such adaptations are not to be con- 
sidered as opposed to broad breeding, because of the many cases 
where the same flower has two kinds of adaptations, some favorable 
to broad breeding and others to self-fertilization, but both tending 
to prevent narrow breeding. Thus it appears that vegetative propa- 
gation and other natural forms of line breeding are to be considered 
as supplementary to broad breeding rather than as substitutes for 
broad breeding. 

The superiority of line breeding over narrow breeding depends on 
the factor of longevity in the protoplasm, to enable the growth of new 
individuals to continue without the need of frequent recourse to the 
physiological stimulus of conjugation. Varieties having the necessary 
longevity are propagated more successfully b}^ line breeding than by 
narrow breeding, showing that protoplasmic longevity is able to 
sustain the vitality of such stocks better than the conjugations which 
occur under conditions of narrow breeding without adequate diversity 
of descent. 

The superiority of line breeding over narrow breeding explains the 
improvement often wrought by closer selection in narrow-bred groups. 
Varieties may be improved by more rigid selection for uniformity 
if they are thus carried from the condition of narrow breeding toward 
the more favorable condition of line breeding. Varieties may be in- 
jured by more rigid selection when it carries them from broad breed- 
ing to narrow breeding, especially those varieties which can not be 
placed on a basis of line breeding for lack of the necessary longevity. 

The effect of line breeding is to restrict the expression of characters 
to a single individual set by suppressing the original diversity of the 
group. Nevertheless, the suppressed characters have a persistent 
tendency to return to expression, especially when opportunities are 
afforded by crossings between different lines of descent or by changes 
in external conditions. There is no warrant for believing that any 
method of selection can establish varieties on a stable basis so as to 
prevent the return of diversity and render further selection unneces- 
sary. Selection always appears to improve narrow-bred and line- 
bred varieties, not because it raises them to new standards but because 
it weeds out those lines of descent which have failed to maintain the 
old standards. 

The fact that different physiological principles are involved in the 
different methods of reproduction practiced in the various kinds of 

146 



40 LINE BREEDING AND NARROW BREEDING. 

plants and animals shows that no single generalization can be applied 
to the whole field of practical breeding. The success of line breeding 
in some cases does not warrant the advice that line breeding be ap- 
plied to all cases, nor do particular failures with line breeding justify 
any general insistence that crossing must be practiced in all varieties. 
The practical need is to recognize the effects that the different methods 
of breeding are exerting upon our varieties, so that we may -guard 
them against deterioration as long as possible and provide other 
varieties to replace them. 

146 



INDEX 



Page. 

Amitapsis distinguished from partlienogenesis 17 

iu Oenotliera 17 

Australia, sheep breeding 32 

Beans, correlated characters 33 

Biotypes originated by mutation ,26 

Birds carrying pollen — _ 19 

Breeding, balance between broad and narrow 31-32 

broad, adaptations 19 

and line breeding in same species 19-20 

defined 8-37 

supplemented by self-fertilization in natural species 18-19 

centgener method 21, 36-37 

constructive and conservative 25 

ci'oss, definition 8 

importance of longevity 39 

in-and-in, allied to vegetative propagation 14 

definition 8-9, 17-18 

line, a system of propagation 20-23 

adaptations 19 

and broad breeding iu same species 19-20 

by vegetative propagation 11 

definition S-0, 37 

improvement of varieties by selection 24 

in nature 15 

methods 14—15 

recurrence of diversity in crosses between groups 36-37 

superiority 38 

to fix characters 35 

vegetative propagation the most effective method 11 

methods defined 9-37 

essentially diverse *40 

narrow, and mass selection 29 

avoided in nature 19 

definition 8-9,37 

physiological principles 39 

to fix characters 35 

Bud mutations 22 

infrequent 23 

more frequent in new varieties 23 

of coffee 23 

Canary Island date palm 19 

Carnations, "running out" of varieties 29-30 

Cells, simple and double 13 

Centgener method of breeding 21,36-37 

146 

41 



42 LINE BREEDING AND NARROW BREEDING. 

Page. 

Central America, coffee mutations 23 

Cereals, origin of self-fertilization 15 

Characters, correlated 33 

expression in line-bred groups 36 

restricted by line breeding 39 

"fixing" by line breeding 35 

lettuce, correlated 33 

new combinations 25 

not result of breeding 25 

Coffee, mutations 23 

Conclusions of bulletin 37 40 

Conjugation as means of rejuvenation 12 

prolonged in vegetative propagation 12,13 

self-fertilized, not sexual 14 

Corn, correlated characters 33 

diversity 33 

hermaphrodite flowers 1 35 

hybrid, increased yield 35 

metamerism 34 

Corollas specialized to secure cross-fertilization 20 

Cotton, correlated characters 33 

hybrids, characters 37 

diversity 37 

Mexican, variation in yield 21 

selection in Guatemala 28 

Triumph, observations 27 

unnecessary uniformity 32 

Criminals as examples of degeneration 24 

Cross-breeding, definition 8 

fertilization contrasted with self-fertilization 16 

specialization to secure 20 

Crosses between line-bred groups, recurrence of diversity 36-37 

Darwin, experiments on self-fertilization 15 

on different forms of selection 28 

Date palms, vegetative shoots 19 

De Vries, doctrines 26 

Degeneration as result of breeding 30 

in narrow breeding 10,37 

Bescent as a network 9,37 

in natural species 9-38 

restriction, effects 9-11 

Diversity in cotton hybrids ' 37 

individual crosses 36 

maize 33 

recurrence in crosses between line-bred groups 36 

Domestication of plants 15 

East, E. M., on bud mutations 23 

Evolution affected by restriction of descent 10 

Fertility as standard of superiority 10 

Fertilization, cross, contrasted with self-fertilization , 16 

six'cialixation to secure 20 

Flowers, hermapluodite, in rorn 35 

146 



INDEX. 43 

Page. 

Gates, R. R., studies of Oenothera 16 

Guatemala, method of corn planting 35 

Heredity, uniformity not a normal condition 27-2S 

Hermaphrodite flowers in corn 35 

Hybrids, corn, increased yield 35 

cotton, characters 37 

diversity 37 

more uniform than cross-bred individuals 16 

vigor 31 

Improvement of plants by securing uniformity 11 

Inbreeding, definition 8 

Insect pollination prevented by unfavorable weather 15 

Insects carrying pollen 19 

Internodes, individuality 34 

Introduction to bulletin 7 

Java, sugar-cane experiments 23 

Lanham, Md., experiment _J 34 

Lettuce, correlated characters 33 

Lilies, "running out" of varieties 29 

Lockhart, Tex., observations of cotton 27 

Longevity a factor in ability to withstand self-fertilization IS 

importance in breeding IS, 39 

in vegetative propagation 11-13 

of trees .and herbs 13 

protoplasmic ; 32 

stages ^ 12 

standards . 12 

Maize. S'ee Corn. 

Mebane, A., originator of Triumph cotton 27 

Metamerism in corn 34 

Mexican cotton, variation in yield 21 

Mitapsis absent in Oenothera 17 

Mutations evidence of instability 35 

of coffee -• 23 

vigor 31 

Oenothera, absence of mitapsis 17 

reproduction 16 

Organisms, comparison with clocks _ — 13 

Palms, date, vegetative shoots 19 

Parthenogenesis S-38 

and vegetable propagation 14 

compared to amitapsis 17 

Plants and animals, wild types, extinction 30 

asexual reproduction prevalent in early-blooming species 15 

domestication -^ 15 

improvement by securing uniformity — 11 

line-bred varieties, improvement by selection 24 

parthetiogeuetic 15 

perennial, first domesticated 15 

reproduction, three principal types S-9 

superiority, different kinds ' 10-11 

146 



44 LINE BEEEDING AND NARROW BREEDING. 

Page. 
Plants, varieties, " running out " 29-30 

seedless, value 10 

seedling, stronger 31 

Potatoes, " running out " of varieties 29 

Propagation, line breeding a system 20-23 

vegetative 8 

allied to parthenogenesis 14 

and longevity 11 

as method of line breeding 11 

depends on longevity 11-13 

relation of self-fertility 15-17 

sexuality 12 

suppression of characters 36 

the most effective method of line breeding 11 

Proterandry in maize 34 

Quezaltenango, Guatemala, corn culture by Indians 35 

Rejuvenescence of varieties 30-31 

Reproduction, asexual, prevalent in early-blooming species 15 

duality in methods 18 

three principal types 8-9 

Root crops, tropical, antiquity 11 

■"Running out" of varieties 29 

San Antonio, Tex., experiment . 21 

Selection a conservative process 24-27 

by Indians of Guatemala 28 

continued, necessity 27 

Darwin's distinctions 28 

for earliness in cotton 28 

fertility 21 

improvement 11 

in degenerate varieties 24 

" pure lines " 21-22 

individual 28-29 

limits 21-30 

mass, and narrow breeding 29 

two forms 28-29 

methodical 28 

most beneficial in self-fertilized varieties 26 

natural 28 

of line-bred varieties 24 

produces uniformity 36 

"pure line," difficulties 22 

result mathematical 24 

safest vA'heu not too closely restricted 22 

suppresses characters 35 

unconscious 28 

Self-fertility, relation to vegetative propagation 15-17 

fertilization 8 

advantages 19 

allied to vegetative propagation 14 

cause of uniformity 36 

contrast between corn and wheat 34 

146 



INDEX. 45 



Self-fertilization, contrasted with cross-fertilization 16 

developed in cereals 15 

exclusive, rare 20 

function 20 

in maize ^^ 

natural species 18-19 

like vegetative propagation 15 

postpones degeneration 16 

selection ^^ 

supplement to broad breeding 18 

Sex, method of reproduction 38 

Sexuality in self-fertilizing species 20 

vegetative propagation 12 

Shamel, A, D., statement ^^ 

tobacco studies 1^ 

Sheep as example of balanced breeding 32 

Species a biological entity ^ 

elementary "^ 



extinction. 



30 



Sterility, biological and agricultural fertility 10 

Strawberries, "running out" of varieties 29 

Sugar cane bettered by change 23 

"running out" of varieties 29 

Synapsis in Oenothera 1^ 

Tobacco, example of self-fertilization 15 

Tracy, W. W., sr., statements 33 

Uniformity an agricultural improvement H 

lack, in progeny of single individual 22 

leads to deterioration 38 

limitations -'' 

maintenance by selection 24 

not a normal condition of heredity 27-28 

unnecessary, avoidance 32-35 

Varieties, degenerate, of agricultural value 25 

selection 24 

rejuvenescence 30-ol 

" running out " 29-30 

weakening 13 

Vigor as standard of superiority 10 

increased in hybrids 31 

Vitality and longevity J-* 

146 



i-uoi iJ9 



jj [Continued from page 2 of cover.] 

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